1. Field of the Invention
The invention relates to a disc player for recording and reproducing information to and from a recording disc.
2. Description of Related Art
Nowadays, DVD-R as a WORM (write-once) type DVD (Digital Versatile Disc) and DVD-R/W (or DVD-RAM) as a rewritable DVD are put into practical use.
FIG. 1 is a diagram showing a recording surface of a DVD such as a DVD-R or DVD-R/W in which information data can be recorded by a user.
As shown in FIG. 1, grooves GV and lands LD serving as recording tracks are alternately formed spirally or concentrically on the recording surface of the DVD. In this configuration, a pit train representing the information data (audio data, video data, and computer data) is formed on the groove GV. Land prepits LPP have previously been intermittently formed on the lands LD (before the information data is written). The land prepits LPP are pits which indicate address information to recognize the position on the disc and are used to perform a phase control when the information recording is performed.
To record information data to the DVD-R or DVD-R/W (hereinafter, simply referred to as a recording disc), it is necessary to read the recording information from the recording disc, detect the land prepits LPP, and recognize the position at the time of recording.
Therefore, DVD players for recording and reproducing information data to/from the recording disc are provided with a prepit detecting circuit to detect the land prepits LPP based on a read signal read out from the recording disc.
FIG. 2 is a diagram showing the structure of such a prepit detecting circuit.
In FIG. 2, a 4-split photodetector 20 is constituted by four independent photoelectric converting elements 20a to 20d. These elements 20a to 2d have functions of receiving reflection light by an information reading spot irradiated from a reading laser beam generator (not shown) toward the recording surface of the recording disc, converting it into electric signals, and outputting them as read signals Ra to Rd. A photosensitive surface of the 4-split photodetector is divided into four surfaces in the direction along the recording tracks (grooves GV) of the recording disc and the direction perpendicular to the recording tracks and the divided four photosensitive surfaces are arranged on the photoelectric converting elements 20a to 20d, respectively.
An adder 51 in a prepit detecting circuit 50 adds the read signals Rb and Rc from the photoelectric converting elements 20b and 20c, obtains a summed read signal (Rb+Rc), and supplies it to a subtractor 52. An adder 53 adds the read signals Ra and Rd from the photoelectric converting elements 20a and 20d, obtains a summed read signal (Ra+Rd), and supplies it to the subtractor 52. The subtractor 52 obtains a value by subtracting the summed read signal (Rb+Rc) from the summed read signal (Ra+Rd) and supplies it as a differential value SB to a binarizing circuit 54. The binarizing circuit 54 binarizes the differential value SB based on a predetermined threshold value and generates the binarized value as a prepit detection signal PPD.
FIG. 3 is a diagram showing the read signals Ra to Rd obtained when an information reading spot SP traces the land prepit LPP formed on the recording disc in a state as shown in FIG. 4 and also showing the internal operation of the prepit detecting circuit 50.
When the read signals Ra to Rd as shown in FIG. 3 are supplied to the prepit detecting circuit 50, the subtractor 52 generates the differential value SB whose level rises only for a tracing period of time of the land prepit LPP. The binarizing circuit 54 binarizes the differential value SB by a threshold value SL as shown in the diagram, thereby generating the prepit detection signal PPD which is set to the logic level xe2x80x9c1xe2x80x9d only for the tracing period of time of the land prepit LPP.
In short, by using the structure as shown in FIG. 2, the prepit detecting circuit 50 detects the land prepits LPP on the basis of the read signals Ra to Rd.
In the structure shown in FIG. 2, however, the prepit cannot be correctly detected unless phases and amplitude levels of the read signals Ra to Rd are not matched each other.
For example, if a phase advance xcex94t occurs in only the read signal Ra among the read signals Ra to Rd as shown in FIG. 5, the differential value SB based on the read signals Ra to Rd causes a level increase even in a period of time during which the land prepit LPP is not traced. If it is binarized by the threshold value SL, therefore, as shown in FIG. 5, the prepit detection signal PPD at the logic level xe2x80x9c1xe2x80x9d is erroneously generated even for the period of time during which the land prepit LPP is not traced.
The operation similar to that mentioned above is performed also in cases in which, for example, an amplitude of the read signal Ra among the read signals Ra to Rd is lower than those of the other read signals as shown in FIG. 6.
The invention has been made in view of the above problems and it is an object of the invention to provide a disc player having a prepit detecting circuit which can correctly detect a prepit even if phases or amplitude levels of a plurality of read signals read out from a recording disc are deviated from each other in a way that is not acceptable to conventional prepit detection circuits.
According to the invention, there is provided a disc player comprising: reading means for obtaining a read signal by reading recording information from a recording disc; and a prepit detecting circuit for detecting prepits which have previously been formed on the recording disc based on the read signal and generating a prepit detection signal, wherein the reading means includes first and second reading elements arranged in two regions divided along the direction of recording tracks of the recording disc, and the prepit detecting circuit comprises phase detecting means for detecting a phase difference between a first read signal read out by the first reading element and a second read signal read out by the second reading element, phase correcting means for obtaining a first phase correction read signal by correcting the phase of the first read signal by an amount corresponding to the phase difference, and a subtractor for generating a differential value between the first phase correction read signal and the second read signal as a prepit detection signal.